The present invention relates generally to electronically-controlled flow regulators of the type used in automotive vehicles equipped with computer-controlled emission control systems.
As is known, virtually all modern automotive vehicles are equipped with emission control systems that are operable for limiting the emission of hydrocarbons into the atmosphere. Such emission control systems typically include an evaporative emission control system which traps fuel vapors from the fuel tank in a carbon-filled canister and a purge system which draws the vapors from the canister into the engine intake system. In this manner, fuel vapors from the fuel tank are delivered to the engine for subsequent combustion.
Conventional evaporative emission control systems are equipped with electronically controlled purge valves for regulating the flow rate of fuel vapors introduced into the intake system in response to specific engine operating parameters. Conventional purge valves comprise pulse width modulated (PWM) solenoid valves which are responsive to a duty cycle control signal from the engine computer. However, PWM purge valves provide uneven flow characteristics, particularly at low engine speeds, and also do not provide consistent flow control independent of variations in manifold vacuum.
In view of increasingly stringent emission regulations, the demands on the evaporative emission control system have increased dramatically. In particular, in order to satisfy current EPA emission requirements, the flow capability of the evaporative emission system must be increased. To achieve this result within the EPA city test cycle, it is therefore necessary to provide purge flow at engine idle speeds. Moreover, purge flow control must also be accurately regulated so as not to cause unacceptable excursions in overall engine output emissions.
To provide such enhanced flow control, it is desirable to have the output flow characteristics of the purge valve be proportional to the duty cycle of the electric control signal applied to the valve, even at low engine speeds, and yet be independent of variations in the manifold vacuum. Accordingly, the output flow of the valve should be substantially constant at a given duty cycle control signal and be controllable in response to regulated changes in the duty cycle regardless of variations in manifold vacuum. Moreover, it is also desirable that the output flow of the valve vary substantially linearly from a predetermined "minimum" flow rate at a "start-to-open" duty cycle to a specified "maximum" flow rate at 100% duty cycle.
The above performance demands have prompted the recent development of a purge flow regulator that combines an electric vacuum regulator (EVR) solenoid valve with a diaphragm-type vacuum regulator valve to provide the desired continuous controlled flow characteristics independent of variations in manifold vacuum. In particular, the EVR solenoid valve is connected to the diaphragm vacuum regulator valve so as to regulate the vacuum signal supplied to the reference side of the diaphragm valve in accordance with the control signal from the engine computer.
A closure member, associated with the opposite side of the diaphragm, controls flow from the input port to the output port of the vacuum regulator valve in response to regulated movement of the diaphragm. Since the EVR valve is in communication with atmosphere and a vacuum source, such as the intake manifold of the engine, the amount of vacuum (i.e., the vacuum signal) provided to the reference side of the diaphragm is proportional to the electric control signal supplied to the EVR valve by the on-board engine control computer. Thus, output flow through the vacuum regulator valve is controlled by the duty cycle of the control signal applied to the EVR valve.
Examples of electronically controlled flow purge regulators of this type are disclosed in U.S. Pat. No. 4,534,378 to Cook and U.S. Pat. No. 5,050,568 to Fox. However, for such conventional flow regulators to satisfy the above-described performance specifications, the purge flow regulator must be precisely calibrated. It has been proposed to calibrate the purge flow regulator by adjusting the characteristics of the EVR solenoid valve. In particular, the preload on the armature bias spring of the EVR valve is adjusted for setting the minimum flow rate at the "start-to-open" duty cycle. Such changes in the magnitude of preload on the armature bias spring effectively displaces the performance curve without changing its slope. In addition, the reluctance of the solenoid flux path is adjusted for setting the maximum flow rate at the 100% duty cycle. However, changes in reluctance result in a corresponding change in the slope of the performance curve. As can be appreciated, this calibration approach is problematic in that each adjustment affects the other, such that the two calibration adjustments are dependent and cumulative in nature. As such, it typically requires several iterations to "zero-in" on both of the desired calibration points. Accordingly, while such conventional flow regulators are generally successful in automotive emission control systems for their intended purpose, there is a continuing need to develop alternatives which meet the above-noted performance specifications and can be manufactured and calibrated in a more efficient and cost effective manner.